Method and apparatus for testing telephone circuits



Dec. i1, 192s.

W. WOLFF y METHODAND APPARATUS TESTING TELEPHONE CIRCUITS Filed April24, 1926 Pig. 2.

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Receiving 5h: fion` Patented Dec. 1l, 1928i.

A UNITED STATES PATENT oFFlcE.

COMPANY, A CORPORATIOlQ OF NEVI YORK.

METHOD AND APPAEIATUS FOR TESTING TELEPHONE CIRCUITS.

Application filed April 24, `1.926, Serial No. 104,492, and in 4GermanyMay 2, 1925i..

This invention relates to electrical testing systems, and particularlyto amethod and arrangements in such systems for measuring theattenuationin telephone lines.

Arrangements for measuring the attenuaLV tion of telephone linesare'well-known.l Suchv arrangements are described inan article by F. H.Best in the Transactions ofthe American Institute of ElectricalEngineers for February,l921l, beginning at pagel25 of that article. Onearrangement is shown diagrammatically in Figi) of that article, and thisarrangement is fairly complicated, as will be obvious from thedescription relating thereto. The important draw-back of this arrange`ment is that the impedance of the receiving circuit, -as stated on page426, must be equal to the" impedance of the sending circuit, and that,in order to obtain 4practical values, bothv the impedance of thereceiving circuit and the impedance of the sending lcircuit must beequal tothe impedance of the telephone line vat the n'ioment `that it isbeing measured. That arrangement includes switching means for connectingat willany one of three'fiXed impedances, namely, 600, 1200and lSOOohms,these threeA values being merelythe mean values of the types oftelephone lines that are most frequently used` in practice. The im-,-

pedance of aftelephone line in general deviates somewhat from thesevalues. Furthermore,

the impedance of every telephone line is also dependent upon thefrequency transmitted thereo'ver. Consequently, when higher frequenciesare used, there are errors in ythe measurements which are dueto thedeviations between the impedance of the telephone line to be measuredand the impedance ofthe sending or receiving circuit. y n

The present invention provides an arrangement which makes it possible tomeasurein a very simple manner corr-ect valuesl for .the impedance of a;telephone line.- Moreoven the arrangement of this` invention issubstantially simpler than that of the descrip tion above referred to.yForinstance, the

calibrating network mentioned inthe above article is quite complicatedyso far as anali-.fof the ratio of the'characteristic impedances bratingnet work itself isconcerned,the cali brating` network being theYmeasuring Yapparatus whichis-adjusted for 'every measurement. Y

Y, The features ofmy invention whichare be-Y lowing when read inconnection with the accompanying drawing in which Fig. l represents anetwork which is the equivalent of ya four-terminal network composed ofelements of resistance, and Fig. 2 represents a specific embodiment ofthe invention.

Referring to Figi, there is shown a fourte-rmina-l network includingresistances R1, R, and R3 arranged so as to form a T network. Theseresistances may be adjusted so asto' have any desired attenuation. Letitbe assumed that, when looking at vthe network from the side l,the'network has the characteristic impedance Z1 and that, when lookingat the network from the side 2, the network has the .characteristicimpedanceV ZZQ Asis well known in the art, thek characteristic impedanceZlinay be made equal to thecharacteristic impedance Z2. If this networkis closed onside 2 by means of an impedance which vis equal vto itscorresponding characi teristic impedance, namely, the impedance Z2, theimpedance of the network measured from the side l will be equal to" thech'ar'acfv teristicr impedance correspondingto thatside, namely, theimpedance Z1. Moreover, when a voltage is impressed on the terminals ofthe side l, no reflection will be produced atfthe sid-e 2,since-.thenetwork is terminated by its characteristic impedance. The.current and voltage `distribution of the network closedlin ai mannerdescribed herein will be thesame 'as if the network possessed an in`finitely great attenuation. Consequently,jif

be the attenuationof the network, J 1and J2 andfV,L and V2" thevaluesofthe currentand voltage at the input andthe output terminals ofthenetwork, respectively, the` ratiofofthe current and ,voltagel at vthe. input terminals at its input terminals and its output termi- ,nalsl,TheV ratio, however, is independent of the magnitudes 4of thecharacteristic imv pedances of the fnetwork itself. In the case' ofgasymmetrical networlni. e., a network in ioo which the characteristicimpedances are equal,

In this case the attenuation b depends only on the ratio that thecurrent or voltage at the input terminals of the network bears to thecurrent or voltage at the output terminals of the network. Theattenuation of two networks of any characteristic impedances maytherefore be compared, in accordance with this invention, by terminatingthe two networks in the manner described hereinabove and by applying thesame voltages or currents to their input terminals.

Fig. 2 shows a specific embodiment of the invention particularly ada tedfor the measurement of attenuation o a telephone line. G1 andG2 are twoalternating current generators which supply pure sinusoidal waves freefrom distortion. The generator G2 is at the transmitting station, andthe generator G2 is at the receiving station. A telephone line L, shownin dotted lines, is connected to the generator Gr1 through a shieldedtrans former T1. A calibrating network N of adjustable attenuation isconnected to the gen-l erator G2 through a shielded transformer T2. ThisCalibrating network N comprises resistances R5, R6, R2 and R2 in seriesrelationship with the secondary winding of the transformer T2 and aresistance R2 in shunt relationship with the secondary winding of thetransformer T2. This network is shown comprising pure resistances but itwill be understood that the network may also be made to includeinductances and capacities of suitable magnitudes. There are included inthe secondary windings of the transformers T,L and T2 meters Ml and M2,respectively, these meters being used to aid in adjusting the current orvoltage at the input terminals of the telephone line L and theCalibrating network N, respectively, to the same values.

Bridged across the input terminalsof the telephone line is arnetwork Nlwhich has an impedance equal to the characteristic impedance oftheCalibrating networkN. Thus this network N1 is included between thesecondary Vwinding of the transformer T1 and the telephone line L.Bridged across the input terminals of the Calibrating network N is anetwork N2 which has an impedance equal to the characteristic impedanceZ1 otthe telephone line L when looking at the line from the transmittingstation. Thus this network N2 is connected between the secondary windingof the transformer T2 and the Calibrating network N. Furthermore, anetwork`N2 having a characteristic impedance of the telephone line isbridged across the output terminals of the telephone line L so that thetelephone line is terminated by its own characteristic impedance; and anet- ,put terminals r2-7),.

work N1 having a characteristic impedance of the Calibrating network Nis bridged across the output terminals of the calibrating network N, sothat the calibrating network is terminated by its own characteristicimpedance. y y

A switch S 1s arranged so as to connect either the output terminals ofthe telephone line L or the output terminals of the calibrating networkN to an indicating meter V or other measuring device of high impedance.`This indicating meter V is used for comparing the conditions at theoutput terminals of the telephone line L with those at the outputterminals of the Calibrating network N. i

The switch S is moved iirst so as to connect the indicating meter V withthe output terminals of the telephone line L. The generator G1 transmitsa current through the transformer T1 tothe input terminals L1-b1 of thetelephone line L. The pointer of the indicating meter V will then bedeflected and its deflection may be read. The exact value of the drop inpotential over the telephone line need not yet be known. The switch S isthen moved so as to connect the output terminals of the Calibratingnetwork N with the indicating meter V. The generator G2 then supplies acurrent to the input terminals t2-b2 of the Calibrating network Nthrough the transformer T2. The current transmitted by the generator G2is then adjusted until the current flowing to the input terminals of thecalibrating network N equals that which previously flowed to the inputtermi nals oi' the telephone line L. In other words, the currentsareadjusted until the meter M2 reads exactly the same as the meter M1reads, thus making the conditions at the input terminals a2--b2 the sameas those at the in- The Calibrating network N is then adjusted until thedeflection of the indicating meter V equals the deflection of that meterwhen the switch-S was connected to the output terminals of the telephoneline L. When this adjustment is made, the conditions at the outputterminals of the Calibrating network N are made the same as theconditions at 'the output termi nals of the telephone line L. Since thecalibrating network N and the telephone line L may be said to have theproperties of infinitely long lines because they are terminated by theirown characteristic impedances, then the attenuation as determined fromthe calibrating networkN will be equal to the total attenuation of thetelephone line L without regard to the characteristic impedance of thetelephone line L or the characteristic impedance of' the Calibratingnetwork N.

It seems hardly necessary to state that when the loss of the network Nis large,'the resistance R, is small and forms only a small part ofthecharacteristic impedance ofthe las network N. The resistance R9 may bevaried over a considerable range without materially aii'ecting themagnitude of thecharacteristic impedance of network N. Thus, theresistance N, has a magnitude always substantially equal to thecharacteristic impedance of network N, even when the impedance of thisnetwork is varied by the manipulation of the resistance R9.

It will be understood, however, that although the arrangements of thisinvention have been described particularly'with reference to theadjustment of voltage conditions at the input and output terminals ofboth the telephone line and the Calibrating network, respectively, thesame arrangements may be employed for adjusting the current conditionsot both the telephone line and the Calibrating network.

ldhile the arrangements of this invention have been shown and describedwith respect to the particular embodiment of Fig. 2, it is to beunderstood that the invention is capable ot embodiment in other andwidely Varied organizations without departing from the spirit ot theinvention and the scope of the appended claims.`

llVhat I claim as new and desireto secure by Letters Patent of theUnited States, is:

l. The method of determining the attenuation of a telephone line with anadjustable artificial network and suitable impedance elements, whichconsists in setting up a predetermined electrical condition across theinput terminals of a telephone line, setting up the same electricalcondition across the input terminals of the artificial network,terminating said telephone line and said artificial network by theirrespective characteristic impedances, adjusting the attenuation of thearticial line to equal the attenuation of the telephone line, andobserving at the output terminals of the telephone line and at theoutput terminals or' the artificial network when the attenuation of theartificial line equals the attenuation of the telephoneline.

2. The method oi determining the attenuation of a telephone line withapparatus including an artificial network, the input and outputterminals ot the telephone line being respectively shunted by thecharacteristic impedance of the artificial vnetwork and thecharacteristic impedance of the telephone line, the input and out-putterminals of the artiiicial network being respectively shunted by thecharacteristic impedance yof the telephone line and the characteristicimpedance of the artiiicial network, which consists in setting up apredetermined voltage across the input terminals of the telephone line,setting up the same voltage across the input terminals of the artificialnetwork, adjusting the attenuation of the artificial line to equal theattenuation of the telephone line, and observing when the attenuation otthe artificial line equals the attenuation 0i the telephone line.

3. A system for measuring the attenuation of a telephone line comprisingan artificial network, means for setting up the same voltages across theinput terminals of the telephone line and the input terminals of theartiiicial network, the telephone line being terminated by itscorresponding characteristic impedance, the vartiiicial network beingterminated by its corresponding characteristic impedance, means foradjusting the attenuation of the artiiicial line to equal theattenuation of the telephone line, and means for observing when theattenuation of the `artificial line equals the attenuation of thetelephone line. v

4. In combination, a telephone line, an articial network, two networkseach having the characteristic impedance of the artiiicial line, onebridging the input terminals of the telephone line and the otherbridging the output terminals of the artilicial vline, two networks eachhaving the characteristic inl-- pedance et the telephone line, one.bridging the output terminals of the telephone line and the otherbridging the input terminals ot the artificialnetwork, means forimpressing the same currents on the input terminals oli-the telephoneline and on the input terminals of the artificial network, means foradjusting the attenuation of the artiiicial network to equal theattenuation oi the telephone line, and means for indicating the equalityof the attenuations of the telephone line and the artiiicial network. Y

5. ln combination, a telephone' line, an artiiicial network, twonetworks each having the characteristic impedance of the artiiicialline, one bridging the input terminals of the telephone line and theother bridging the output terminals of the artificial line, two networkseach having the characteristic impedance of the telephone line, onebridging the output terminals ot the telephone line and theother-bridging the input terminals oi."- the artiiicial line, twoalternating current generators each so arranged as to transmit equalcurrents through f the t-elephone line and through the artiiicialnetwork, means for adjusting' the attenuation ot the'artiiicial networkto equal theattenuation vof the telephone line, and a measuring deviceof high impedance for indicating when the attenuation of the artiiicialnetwork equals the attenuation of the telephone line.

ln witness whereof, l have hereunto set y myhand this 15th day of March,1926;

` l wALrnER woLrr,

